In any transmission system based on digital modulation, transmission may be disturbed by disturbing signals as understood in a wide sense, i.e.:
either by other transmission systems; or
by deliberate jamming.
One prior art solution for detecting such disturbing signals as described in French patent No. 2 538 645 (application number 82 21 875, filed Dec. 28, 1982), consists in detecting the energy received outside the modulation band by a highpass filter having a cutoff frequency that is not less than the highest frequency of the modulation band. However, although such apparatus is effective for broadband disturbing signals, it functions poorly with narrow band disturbing signals. Under such circumstances, when a signal of adjacent frequency is detected, the received signal is taken to be disturbed even though it is not.